One example of an axial flow rotary machine is a gas turbine engine for powering an aircraft. Such an engine includes a rotor assembly which extends axially through the engine. A stator assembly is spaced radially from the rotor assembly and includes an engine case which circumscribes the rotor assembly. A flow path for working medium gases is inward of the case. The flowpath extends axially through the engine between the stator assembly and the rotor assembly.
The rotor assembly includes rows or arrays of rotor blades. The rows of rotor blades extend radially outward across the working medium flow path into proximity with the case. Arrays of stator vane assemblies are interdergitated with the arrays of rotor blades. The stator vanes extend inward from the case across the working medium flow path into proximity with the rotor assembly to guide the working medium gases as the gases are discharged from the rotor blades.
A stator vane assembly includes a stator vane and an inner air seal which is attached to the inner ends of the stator vanes. The inner air seal extends in close proximity to the rotor assembly to block the leakage of working medium gases from the working medium flow path. One example of such a construction is shown in U.S. Pat. No. 4,431,373 entitled "Flow Directing Assembly for a Gas Turbine Engine" issued to William J. Monsarrat. In this patent, the stator assembly includes an outer case and an inner case which extend circumferentially about the working medium flowpath. Each stator vane is attached at its outer end to the inner case and is joined at its inner end by an inner air seal. The inner air seal is integrally attached to the vanes and extends circumferentially about the rotor assembly. The inner air seal is segmented to accommodate changes in the diameter of the inner air seal which occur as a result of radial movement of the inner case.
Another example of an inner air seal construction is shown in French Patent 2,404,102 entitled "Couronne d'aubage fixe pour turbomachine axiale et son procede de realisation" issued to Claude Dubois. This patent shows an array of stator vanes extending inward from a case. The vanes are integrally attached at their base to the case. The inner ends of the vanes are attached to an inner air seal. The inner air seal has a U-shaped channel which is adapted by elongated holes to receive a tongue on each stator vane. The channel is filled with an abradable sealing material which faces the rotor assembly and provides a sealing surface for the rotor assembly. The abradable material also attaches the U-shaped channel to the stator vane and provides a damping medium for damping vibrations in the stator vane.
Vibrations in the stator vane result from buffeting of the vane by the working medium gases as they pass through the stator vanes and exert aerodynamic forces on the stator vanes. The forces are primarily the result of wakes from the upstream rotor blades and bow waves from the downstream rotor blades and are related to the number of rotor blades in the downstream and upstream rows of rotor blades. Damping is important because it reduces the amplitude of the vibrations which reduces the associated cyclic stresses in the stator vane and insures an adequate fatigue life for the stator vane. These stresses include bending and multimode stresses which result from deflection of the vane and torsional stresses which result from twisting of the vane.
The above art notwithstanding, scientist and engineers working under the direction of Applicants' assignee have sought to reduce vibrations in stator vanes by damping to decrease bending, multimode and torsional stresses in the vane.